Regeneration of spent caustic solutions for treating gasoline



Patented Aug. 12, 1947 ATNT orrics REGENERATION F SPENT CAUSTIC SOLU- TIONS FOR TREATING GASOLINE of Ohio No Drawing. Application August 26, 1944, Serial No. 551,453

3 Claims. 1

Thi invention relates to a method for removing acidic sulfur compounds from hydrocarbon fluids and to a method of regenerating alkali solutions used in removing acidic sulfur compounds from hydrocarbon fluids.

It is well-known to extract acidic sulfur compounds such as mercaptans, from hydrocarbon fluids, such as petroleum oil fractions, by contact with aqueous alkali solution followed by regeneration of the alkali solution by means of steam stripping or air blowing. In order to improve the ability of aqueous alkali solutions to extract acidic sulfur compounds from hydrocarbon fluids, it is common practice to fortify the alkali solution with solubility promoters. Among solubility promoters which have been used are alkyl phenols and naphthenic acids and the alkali metal salts thereof. The presence of solubility promoters of the type above mentioned have the disadvantage of increasing the solubility of gasoline and other low-boiling hydrocarbons in the alkali solution, thereby necessitating means for recovering these hydrocarbons in order to avoid their loss during regeneration of the alkali solution.

An object of my invention is to provide an improved method for removing acidic sulfur compounds from hydrocarbon fluids.

Another object of my invention is to provide an improved method for extracting mercaptans from low-boiling liquid hydrocarbons.

Still another object of my invention is to provide an improved method for regenerating used aqueous alkali solutions resulting from the extraction of acidic sulfur compounds from hydrocarbon fluids.

A further object of the invention is to provide against loss of valuable hydrocarbons during regeneration of used aqueous alkali solutions,

Other objects of my invention will become apparent from the following description.

In accordance with my invention, used alkali from the treatment of hydrocarbon fluids, such as gasoline, containing acidic sulfur compound, such as mercaptans, is contacted with substantially pure oxygen in the presence of an oxidation catalyst which is capable of causing the oxygen to react stoichiometrically with the acidic sulfur compounds present in the used alkali solution in a relatively short period of time.

My process is particularly applicable to regeneration of used alkali solutions containing solubility promoters such as naphthenic acid salts and/or phenolates, since these solutions dissolve substantial quantities of the hydrocarbons treated. I have found that aqueous alkali solutions containing sodium naphthenates and sodium cresolate may dissolve up to approximately 5% by volume of gasoline, which is lost during regeneration by steam stripping or air blowing, unless fractionating and condensing apparatus is provided for recovery thereof.

In accordance with my process, a small amount of oxidation catalyst, particularly pyrogallol or butyl pyrogallol, is dissolved in the aqueous alkali solution. Catalyst may be used in amounts ranging from approximately .05 to 2% by volume. The used alkali solution may be contacted in a bubble tower or in a tower packed with Raschig rings or other contacting surfaces with a countercurrent stream of substantially pure oxygen. The tower should be sufficiently high and the rates of flow of used alkali solution and oxygen should be adjusted to provide sufficient time of contact between the oxygen and used alkali to entirely absorb the oxygen.

The amount of oxygen necessary can be easily determined by first determining the mercaptide sulfur content of the used treating solution. Since 2.8 cubic feet of oxygen is required to convert one pound of mercaptide sulfur to disulfide sulfur, it is a simple matter to calculate the amount of oxygen necessary to reduce the mercaptide sulfur to the required level. In general, it is not desirable to convert all the mercaptide sulfur to disulfide sulfur since in doing so, there is risk of irreversibly oxidizing the oxidation catalyst to an inactive material. In general, the mercaptide sulfur content of the used solution is reduced to approximately .2 to .6% by weight of mercaptide sulfur depending upon the particular oxidation catalyst.

By utilizing substantially pure oxygen, there is no formation of undesirable volatile constituents in the regeneration step and consequently, no loss of gasoline boiling constituents from the used solution. The disulfides which form separate as a supernatant layer above the regenerated aqueous caustic alkali and can be decanted from the system. The disulfides can be disposed of in any suitable way, but are preferably fractionated to remove any hydrocarbons and if desired, to separate the disulfides into close out fractions or individual compounds.

Although substantially pure oxygen alone may be used in the regeneration step, I prefer to mix the oxygen With a low-boiling hydrocarbon which serves as a diluent to prevent or inhibit irreversible oxidation of the oxidation catalyst. As diluent, I may use pentane, cyclopentane, hexane or any mixture thereof or other low-boiling, inert,

water-immiscible liquid which will condense at the temperature of the treating solution during the regeneration stage. When using inflammable liquids, care should be exercised to use a mixture which is non-explosive. For instance, if pentane is used as a diluent, the ratio of pentane to oxygen should exceed one to four. The proper mixture of oxygen and hydrocarbon can be maintained by causing the oxygen to bubble through a body of the liquid at a temperature such that sufiicient liquid will volatilize to give a nonexplosive mixture. In the case of pentane, if regeneration is carried out at atmospheric pressure, the partial pressure of pentane in the nonexplosive mixture should be 152 mm. or higher. Since the vapor pressure of N-pentane is 152 mm. at -4 0., it is necessary only to pass oxygen through a body of pentane maintained above 4 C. to obtain a non-explosive mixture of pentane and oxygen.

- In addition to serving as a diluent, to prevent undesirable oxidation of the oxidation catalyst, the low-boiling liquid also serves as a wash to remove occluded disulfides from the regenerated alkali solution. As previously pointed out, the temperature andpressure in the regeneration zone are regulated and the diluent is selected so that the diluent will condense under the conditions existing within the regenerating zone.

The regeneration may b carried out under atmospheric temperature and pressure conditions or may be carried out under super-atmospheric pressure as well as other temperatures above normal atmospheric pressure, preferably not above approximately 130 F.

As an example of the operation of'the process, used alkali solution originally prepared by mixing together Per cent by weight Sodium hydroxide 18.3 Naphthenic acids 18.7 Cresols 6.3 Water 56.2 Pyrogallol .5

after contact with gasoline to extract mercaptans therefrom, is charged to the upper end of a bubble tower containing approximately trays. The mixture of oxygen and N-pentan prepared by bubbling oxygen through a bath of N-pentane at approximately 20 C. is charged to the bottom of the tower at a rate of approximately 0.6 cubic feet of oxygen per minute. The used alkali solution containing 0.4% of mercaptide sulfur is charged to the tower at temperature of approximately 80 F. and at a rate of approximately 10 gallons per minute. Under these conditions the oxygen is entirely consumed in reducing the mercaptan sulfur content of the alkali solution to 0.2% and they pentane condenses, forming a liquid layer mixed with disulfides. The liquid layer of pentane and disulfide, together with any hydrocarbons which may have been removed from the alkali solution, is withdrawn and fractionated to recover the hydrocarbons. The alkali solution, which may contain from one to five percent of hydrocarbons including some pentane, is recycled to the treating zone for contact with raw untreated gasoline or other hydrocarbon fluid. Pentane recovered from the disulfide fractionation is recycled for further mixture with oxygen used for regeneration.

Although I have mentioned only pyrogallol and butyl pyrogallol as oxidation catalysts, it is to be understood that other oxidation catalysts may be used. However, I prefer pyrogallol and butyl pyrogallol because of their effectiveness in rapidly catalyzing the oxidation of mercaptide sulfur to disulfide sulfur. When using less active catalyst, it may be necessary to provide for longer contact time between the oxygen and the solution undergoing regeneration.

In accordance with my invention, any gasoline or diluent dissolved in the alkali solution is continuously recycled through the process and may eventually be recovered if and when the alkali solution is eventually discarded, by simple fractionation.

It is claimed:

1. The method of regenerating used aqueous alkali solution from the removal of acidic sulfur compounds from hydrocarbon fluids comprising contacting said solution in the presence of an organic oxidizable catalyst comprising a phenolic substance capable of promoting oxidation of said acidic sulfur compounds with a mixture of substantially pure oxygen and vapor of a hydrocarbon which is normally liquid and does not boil at regeneration temperature and pressure and which is Water-immiscible and inert under regeneration conditions; the amount of oxygen in the mixture being just sufllcient to oxidize a sumcient amount of said sulfur compounds to enable said solution to extract further amounts of said sulfur compounds, but insuflicient to irreversibly oxidize said catalyst.

2. Method in accordance with claim 1 in which the substance is a low-boiling hydrocarbon liquid and it is present in the mixture in non-explosive proportions.

3. The method of regenerating used alkali solution containing mercaptides comprising contacting said solution in the presence of an organic oxidizable catalyst comprising a phenolic substance capable of promoting oxidation of mercaptans to disulfides with a mixture composed substantially only of oxygen and vapors of a lowboiling normally liquid hydrocarbon Which does not boil at regeneration temperature and pressure, the amount of oxygen being just sufficient to lower the mercaptide content of the solution sufficiently for further use of said solution but insufiicient to irreversibly oxidize said catalyst and said vapors being present in said mixture in non-explosive proportions.

DONALD C. BOND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,943,744 Rosenstein (A) Jan 16, 1934 1,827,912 Rosenstein (B) Oct. 20, 1931 1,998,863 Chaney et al. Apr. 23, 1935 2,015,038 Pevere Sept. 17, 1935 2,316,753 Ayers et a1 Apr. 20, 1943 2,315,530 Loyd (A) Apr. 6, 1943 2,316,092 Loyd (B) Apr. 6, 1943 

